There are many medical applications where it is desirable to continuously monitor the blood pressure of a patient, i.e., during surgery, while the patient is in intensive care and during stress testing, to name only a few. Oscillometric blood pressure measurement is not suitable for continuous monitoring, since this method can, at best, produce blood pressure measurements at infrequent intervals. In addition, oscillometric blood pressure measurement requires repeated occlusion of an artery, often producing extreme discomfort in the patient.
Methods and apparatus for continuous blood pressure determination are known. U.S. Pat. Nos. 4,669,485 and 4,718,428, for example, disclose systems for continuous blood pressure measurement using a low pressure cuff for continuous monitoring and a high pressure occlusive cuff for periodic calibration of the low pressure cuff. The two cuffs are normally secured around opposite limbs of the patient, although both cuffs can be attached to the same limb. Blood pressure determination is interrupted during the calibration of these systems.
Tonometric blood pressure monitoring systems are also known. The principles of arterial tonometry are disclosed in U.S. Pat. Nos. 3,219,035, 4,799,491 and 4,802,488. These principles are also described in several publications including and article entitled "Tonometry, Arterial," in Volume 4 of the Encyclopedia of Medical Devices and Instruments, John Wiley & Sons 1988. All of these references discuss arterial tonometry as used for the measurement of blood pressure. Tonometric systems alleviate patient discomfort since the tonometric sensor is held against the patient above, for example, a radial artery with enough pressure to flatten the artery but less than that needed to occlude the artery.
While tonometry theoretically provides continuous blood pressure measurements without calibration, potential system errors and artifacts caused, for example, by patient movement make it prudent to include a system for calibrating a tonometric continuous blood pressure monitoring system.